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Find length of the largest region in Boolean Matrix
• Difficulty Level : Medium
• Last Updated : 15 Oct, 2020

Consider a matrix with rows and columns, where each cell contains either a ‘0’ or a ‘1’ and any cell containing a 1 is called a filled cell. Two cells are said to be connected if they are adjacent to each other horizontally, vertically, or diagonally. If one or more filled cells are also connected, they form a region. find the length of the largest region.

Examples:

```Input : M[] = { 0 0 1 1 0
1 0 1 1 0
0 1 0 0 0
0 0 0 0 1 }
Output : 6
In the following example, there are
2 regions one with length 1 and the
other as 6. So largest region: 6

Input : M[] = { 0 0 1 1 0
0 0 1 1 0
0 0 0 0 0
0 0 0 0 1 }
Output: 4
In the following example, there are
2 regions one with length 1 and the
other as 4. So largest region: 4
```

Asked in: Amazon interview

Solution: The idea is based on the problem or finding number of islands in Boolean 2D-matrix

Approach:

1. A cell in 2D matrix can be connected to at most 8 neighbours.
2. So in DFS, make recursive calls for 8 neighbours of that cell.
3. Keep a visited Hash-map to keep track of all visited cells.
4. Also keep track of the visited 1’s in every DFS and update maximum length region.

Below is implementation of above idea.

## C++

 `// Program to find the length of the largest``// region in boolean 2D-matrix``#include ``using` `namespace` `std;``#define ROW 4``#define COL 5` `// A function to check if``// a given cell (row, col)``// can be included in DFS``int` `isSafe(``int` `M[][COL], ``int` `row, ``int` `col,``           ``bool` `visited[][COL])``{``    ``// row number is in range,``    ``// column number is in``    ``// range and value is 1``    ``// and not yet visited``    ``return` `(row >= 0) && (row < ROW) && (col >= 0)``           ``&& (col < COL)``           ``&& (M[row][col] && !visited[row][col]);``}` `// A utility function to``// do DFS for a 2D boolean``// matrix. It only considers``// the 8 neighbours as``// adjacent vertices``void` `DFS(``int` `M[][COL], ``int` `row, ``int` `col,``         ``bool` `visited[][COL], ``int``& count)``{``    ``// These arrays are used``    ``// to get row and column``    ``// numbers of 8 neighbours``    ``// of a given cell``    ``static` `int` `rowNbr[] = { -1, -1, -1, 0, 0, 1, 1, 1 };``    ``static` `int` `colNbr[] = { -1, 0, 1, -1, 1, -1, 0, 1 };` `    ``// Mark this cell as visited``    ``visited[row][col] = ``true``;` `    ``// Recur for all connected neighbours``    ``for` `(``int` `k = 0; k < 8; ++k)``    ``{``        ``if` `(isSafe(M, row + rowNbr[k],``                   ``col + colNbr[k],``                   ``visited))``        ``{``            ``// Increment region length by one``            ``count++;``            ``DFS(M, row + rowNbr[k], col + colNbr[k],``                ``visited, count);``        ``}``    ``}``}` `// The main function that returns``// largest  length region``// of a given boolean 2D matrix``int` `largestRegion(``int` `M[][COL])``{``    ``// Make a bool array to mark visited cells.``    ``// Initially all cells are unvisited``    ``bool` `visited[ROW][COL];``    ``memset``(visited, 0, ``sizeof``(visited));` `    ``// Initialize result as 0 and travesle through the``    ``// all cells of given matrix``    ``int` `result = INT_MIN;``    ``for` `(``int` `i = 0; i < ROW; ++i)``    ``{``        ``for` `(``int` `j = 0; j < COL; ++j)``        ``{``            ``// If a cell with value 1 is not``            ``if` `(M[i][j] && !visited[i][j])``            ``{``                ``// visited yet, then new region found``                ``int` `count = 1;``                ``DFS(M, i, j, visited, count);` `                ``// maximum region``                ``result = max(result, count);``            ``}``        ``}``    ``}``    ``return` `result;``}` `// Driver code``int` `main()``{``    ``int` `M[][COL] = { { 0, 0, 1, 1, 0 },``                     ``{ 1, 0, 1, 1, 0 },``                     ``{ 0, 1, 0, 0, 0 },``                     ``{ 0, 0, 0, 0, 1 } };` `    ``// Function call``    ``cout << largestRegion(M);` `    ``return` `0;``}`

## Java

 `// Java program to find the length of the largest``// region in boolean 2D-matrix``import` `java.io.*;``import` `java.util.*;` `class` `GFG {``    ``static` `int` `ROW, COL, count;` `    ``// A function to check if a given cell (row, col)``    ``// can be included in DFS``    ``static` `boolean` `isSafe(``int``[][] M, ``int` `row, ``int` `col,``                          ``boolean``[][] visited)``    ``{``        ``// row number is in range, column number is in``        ``// range and value is 1 and not yet visited``        ``return` `(``            ``(row >= ``0``) && (row < ROW) && (col >= ``0``)``            ``&& (col < COL)``            ``&& (M[row][col] == ``1` `&& !visited[row][col]));``    ``}` `    ``// A utility function to do DFS for a 2D boolean``    ``// matrix. It only considers the 8 neighbours as``    ``// adjacent vertices``    ``static` `void` `DFS(``int``[][] M, ``int` `row, ``int` `col,``                    ``boolean``[][] visited)``    ``{``        ``// These arrays are used to get row and column``        ``// numbers of 8 neighbours of a given cell``        ``int``[] rowNbr = { -``1``, -``1``, -``1``, ``0``, ``0``, ``1``, ``1``, ``1` `};``        ``int``[] colNbr = { -``1``, ``0``, ``1``, -``1``, ``1``, -``1``, ``0``, ``1` `};` `        ``// Mark this cell as visited``        ``visited[row][col] = ``true``;` `        ``// Recur for all connected neighbours``        ``for` `(``int` `k = ``0``; k < ``8``; k++)``        ``{``            ``if` `(isSafe(M, row + rowNbr[k], col + colNbr[k],``                       ``visited))``            ``{``                ``// increment region length by one``                ``count++;``                ``DFS(M, row + rowNbr[k], col + colNbr[k],``                    ``visited);``            ``}``        ``}``    ``}` `    ``// The main function that returns largest length region``    ``// of a given boolean 2D matrix``    ``static` `int` `largestRegion(``int``[][] M)``    ``{``        ``// Make a boolean array to mark visited cells.``        ``// Initially all cells are unvisited``        ``boolean``[][] visited = ``new` `boolean``[ROW][COL];` `        ``// Initialize result as 0 and traverse through the``        ``// all cells of given matrix``        ``int` `result = ``0``;``        ``for` `(``int` `i = ``0``; i < ROW; i++)``        ``{``            ``for` `(``int` `j = ``0``; j < COL; j++)``            ``{` `                ``// If a cell with value 1 is not``                ``if` `(M[i][j] == ``1` `&& !visited[i][j])``                ``{` `                    ``// visited yet, then new region found``                    ``count = ``1``;``                    ``DFS(M, i, j, visited);` `                    ``// maximum region``                    ``result = Math.max(result, count);``                ``}``            ``}``        ``}``        ``return` `result;``    ``}` `    ``// Driver code``    ``public` `static` `void` `main(String args[])``    ``{``        ``int` `M[][] = { { ``0``, ``0``, ``1``, ``1``, ``0` `},``                      ``{ ``1``, ``0``, ``1``, ``1``, ``0` `},``                      ``{ ``0``, ``1``, ``0``, ``0``, ``0` `},``                      ``{ ``0``, ``0``, ``0``, ``0``, ``1` `} };``        ``ROW = ``4``;``        ``COL = ``5``;``      ` `        ``// Function call``        ``System.out.println(largestRegion(M));``    ``}``}` `// This code is contributed by rachana soma`

## Python3

 `# Python3 program to find the length of the``# largest region in boolean 2D-matrix` `# A function to check if a given cell``# (row, col) can be included in DFS`  `def` `isSafe(M, row, col, visited):``    ``global` `ROW, COL` `    ``# row number is in range, column number is in``    ``# range and value is 1 and not yet visited``    ``return` `((row >``=` `0``) ``and` `(row < ROW) ``and``            ``(col >``=` `0``) ``and` `(col < COL) ``and``            ``(M[row][col] ``and` `not` `visited[row][col]))` `# A utility function to do DFS for a 2D``# boolean matrix. It only considers``# the 8 neighbours as adjacent vertices`  `def` `DFS(M, row, col, visited, count):` `    ``# These arrays are used to get row and column``    ``# numbers of 8 neighbours of a given cell``    ``rowNbr ``=` `[``-``1``, ``-``1``, ``-``1``, ``0``, ``0``, ``1``, ``1``, ``1``]``    ``colNbr ``=` `[``-``1``, ``0``, ``1``, ``-``1``, ``1``, ``-``1``, ``0``, ``1``]` `    ``# Mark this cell as visited``    ``visited[row][col] ``=` `True` `    ``# Recur for all connected neighbours``    ``for` `k ``in` `range``(``8``):``        ``if` `(isSafe(M, row ``+` `rowNbr[k],``                   ``col ``+` `colNbr[k], visited)):` `            ``# increment region length by one``            ``count[``0``] ``+``=` `1``            ``DFS(M, row ``+` `rowNbr[k],``                ``col ``+` `colNbr[k], visited, count)` `# The main function that returns largest``# length region of a given boolean 2D matrix`  `def` `largestRegion(M):``    ``global` `ROW, COL` `    ``# Make a bool array to mark visited cells.``    ``# Initially all cells are unvisited``    ``visited ``=` `[[``0``] ``*` `COL ``for` `i ``in` `range``(ROW)]` `    ``# Initialize result as 0 and travesle``    ``# through the all cells of given matrix``    ``result ``=` `-``999999999999``    ``for` `i ``in` `range``(ROW):``        ``for` `j ``in` `range``(COL):` `            ``# If a cell with value 1 is not``            ``if` `(M[i][j] ``and` `not` `visited[i][j]):` `                ``# visited yet, then new region found``                ``count ``=` `[``1``]``                ``DFS(M, i, j, visited, count)` `                ``# maximum region``                ``result ``=` `max``(result, count[``0``])``    ``return` `result`  `# Driver Code``ROW ``=` `4``COL ``=` `5` `M ``=` `[[``0``, ``0``, ``1``, ``1``, ``0``],``     ``[``1``, ``0``, ``1``, ``1``, ``0``],``     ``[``0``, ``1``, ``0``, ``0``, ``0``],``     ``[``0``, ``0``, ``0``, ``0``, ``1``]]` `# Function call``print``(largestRegion(M))` `# This code is contributed by PranchalK`

## C#

 `// C# program to find the length of``// the largest region in boolean 2D-matrix``using` `System;` `class` `GFG``{``    ``static` `int` `ROW, COL, count;` `    ``// A function to check if a given cell``    ``// (row, col) can be included in DFS``    ``static` `Boolean isSafe(``int``[, ] M, ``int` `row, ``int` `col,``                          ``Boolean[, ] visited)``    ``{``        ``// row number is in range, column number is in``        ``// range and value is 1 and not yet visited``        ``return` `(``            ``(row >= 0) && (row < ROW) && (col >= 0)``            ``&& (col < COL)``            ``&& (M[row, col] == 1 && !visited[row, col]));``    ``}` `    ``// A utility function to do DFS for a 2D boolean``    ``// matrix. It only considers the 8 neighbours as``    ``// adjacent vertices``    ``static` `void` `DFS(``int``[, ] M, ``int` `row, ``int` `col,``                    ``Boolean[, ] visited)``    ``{``        ``// These arrays are used to get row and column``        ``// numbers of 8 neighbours of a given cell``        ``int``[] rowNbr = { -1, -1, -1, 0, 0, 1, 1, 1 };``        ``int``[] colNbr = { -1, 0, 1, -1, 1, -1, 0, 1 };` `        ``// Mark this cell as visited``        ``visited[row, col] = ``true``;` `        ``// Recur for all connected neighbours``        ``for` `(``int` `k = 0; k < 8; k++)``        ``{``            ``if` `(isSafe(M, row + rowNbr[k],``                       ``col + colNbr[k],``                       ``visited))``            ``{``                ``// increment region length by one``                ``count++;``                ``DFS(M, row + rowNbr[k], col + colNbr[k],``                    ``visited);``            ``}``        ``}``    ``}` `    ``// The main function that returns``    ``// largest length region of``    ``// a given boolean 2D matrix``    ``static` `int` `largestRegion(``int``[, ] M)``    ``{``        ``// Make a boolean array to mark visited cells.``        ``// Initially all cells are unvisited``        ``Boolean[, ] visited = ``new` `Boolean[ROW, COL];` `        ``// Initialize result as 0 and traverse``        ``// through the all cells of given matrix``        ``int` `result = 0;``        ``for` `(``int` `i = 0; i < ROW; i++)``        ``{``            ``for` `(``int` `j = 0; j < COL; j++)``            ``{``                ``// If a cell with value 1 is not``                ``if` `(M[i, j] == 1 && !visited[i, j])``                ``{``                    ``// visited yet,``                    ``// then new region found``                    ``count = 1;``                    ``DFS(M, i, j, visited);` `                    ``// maximum region``                    ``result = Math.Max(result, count);``                ``}``            ``}``        ``}``        ``return` `result;``    ``}` `    ``// Driver code``    ``public` `static` `void` `Main(String[] args)``    ``{``        ``int``[, ] M = { { 0, 0, 1, 1, 0 },``                      ``{ 1, 0, 1, 1, 0 },``                      ``{ 0, 1, 0, 0, 0 },``                      ``{ 0, 0, 0, 0, 1 } };``        ``ROW = 4;``        ``COL = 5;``      ` `        ``// Function call``        ``Console.WriteLine(largestRegion(M));``    ``}``}` `// This code is contributed by Rajput-Ji`
Output
`6`

Complexity Analysis:

• Time complexity: O(ROW * COL).
At worst case all the cells will be visited so time complexity is O(ROW * COL).
• Auxiliary Space:O(ROW * COL).
To store the visited nodes O(ROW * COL) space is needed.

This article is contributed by Nishant Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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